What causes the appearance of dark lines along baseboards and under closed doorways?

The appearance of dark lines along baseboards and under closed doorways is primarily caused by filtration soiling. This phenomenon occurs due to air movement throughout a space, which brings airborne particulates and contaminants into contact with the carpet. When air passes over the edges of the carpet, particularly in areas where there is limited airflow, such as at the base of walls and under doors, it can deposit dirt, dust, and other particles. Over time, these deposits accumulate and create noticeable dark lines, emphasizing the contrast between the soiled areas and cleaner sections of the carpet.

Other factors, such as dust accumulation, can contribute to overall carpet soiling but do not specifically explain the localized dark lines associated with filtration soiling. Oil stains typically originate from spills or furniture, whereas carpet wear refers to the degradation of carpet fibers due to foot traffic or friction, which does not directly lead to streaking or line formation along edges. Thus, filtration soiling is the most accurate explanation for the specific pattern observed.